Halim Kusumaatmaja

Anisotropic Drop Morphologies on Corrugated Surfaces

The spreading of liquid drops on surfaces corrugated with micrometer-scale parallel grooves is studied both experimentally and numerically. Because of the surface patterning, the typical final drop shape is no longer spherical. The elongation direction can be either parallel or perpendicular to the direction of the grooves, depending on the initial drop conditions. We interpret this result as a consequence of both the anisotropy of the contact line movement over the surface and the difference in the motion of the advancing and receding contact lines. Parallel to the grooves, we find little hysteresis due to the surface patterning and that the average contact angle approximately conforms to Wenzels law as long as the drop radius is much larger than the typical length scale of the grooves. Perpendicular to the grooves, the contact line can be pinned at the edges of the ridges, leading to large contact angle hysteresis.

The collapse transition on superhydrophobic surfaces

We investigate the transition between the Cassie-Baxter and Wenzel states of a slowly evaporating, micron-scale drop on a superhydrophobic surface. In two dimensions analytical results show that there are two collapse mechanisms. For long posts the drop collapses when it is able to overcome the free-energy barrier presented by the hydrophobic posts. For short posts, as the drop loses volume, its curvature increases allowing it to touch the surface below the posts. We emphasise the importance of the contact line retreating across the surface as the drop becomes smaller: this often preempts the collapse. In a quasi–three-dimensional simulation we find similar behaviour, with the additional feature that the drop can depin from all but the peripheral posts, so that its base resembles an inverted bowl.

Fully Reversible Transition from Wenzel to Cassie—Baxter States on Corrugated Superhydrophobic Surfaces

Liquid drops on textured surfaces show different dynamical behaviors depending on their wetting states. They are extremely mobile when they are supported by composite solid-liquid-air interfaces (Cassie-Baxter state) and immobile when they fully wet the textured surfaces (Wenzel state). By reversibly switching between these two states, it will be possible to achieve control over the fluid dynamics. Unfortunately, these wetting transitions are usually prevented by surface energy barriers. We demonstrate here a new, simple design paradigm consisting of parallel grooves with an appropriate aspect ratio that allows for the controlled, barrierless, reversible switching of the wetting states upon application of electrowetting. We report a direct observation of the barrierless dynamical pathway for the reversible transitions between the Wenzel (collapsed) and Cassie-Baxter (suspended) states and present a theory that accounts for these transitions, including detailed lattice Boltzmann simulations.

Drop dynamics on chemically patterned surfaces

We compare numerical and experimental results exploring the behaviour of liquid drops moving across a surface patterned with hydrophobic and hydrophilic stripes. A lattice Boltzmann algorithm is used to solve the hydrodynamic equations of motion of the drops allowing us to investigate their behaviour as the stripe widths and the wettability contrast are altered. We explain how the motion of the drop is determined by the interplay between the driving force and the variation in surface force as the drop moves between regions of different contact angle and we find that the shape of the drops can undergo large periodic deviations from spherical. When compared, the numerical results agree well with experiments on micron-scale drops moving across substrates patterned by microcontact printing.

Capillary filling in patterned channels.

We show how the capillary filling of microchannels is affected by posts or ridges on the sides of the channels. Ridges perpendicular to the flow direction introduce contact line pinning, which slows, or sometimes prevents, filling, whereas ridges parallel to the flow provide extra surface that may enhance filling. Patterning the microchannel surface with square posts has little effect on the ability of a channel to fill for equilibrium contact angle theta_{e} less than approximately 30 degrees . For theta_{e} greater than approximately 60 degrees , however, even a small number of posts can pin the advancing liquid front.

Contact line dynamics in binary lattice Boltzmann simulations

We show that, when a single relaxation time lattice Boltzmann algorithm is used to solve the hydrodynamic equations of a binary fluid for which the two components have different viscosities, strong spurious velocities in the steady state lead to incorrect results for the equilibrium contact angle. We identify the origins of these spurious currents and demonstrate how the results can be greatly improved by using a lattice Boltzmann method based on a multiple-relaxation-time algorithm. By considering capillary filling we describe the dependence of the advancing contact angle on the interface velocity.

Imbibition through an array of triangular posts

We present and interpret simulation results showing how a fluid moves on a hydrophilic substrate patterned by a square array of triangular posts. We demonstrate that the shape of the posts leads to anisotropic spreading, and discuss how this is influenced by the different ways in which the posts can pin the advancing front.

Modelling capillary filling dynamics using lattice Boltzmann simulations

We investigate the dynamics of capillary filling using two lattice Boltzmann schemes: a liquid-gas model and a binary model. The simulation results are compared to the well-known Washburns law, which predicts that the filled length of the capillary scales with time as l ∝ t1/2. We find that the liquid-gas model does not reproduce Washburns law due to condensation of the gas phase at the interface, which causes the asymptotic behaviour of the capillary penetration to be faster than t1/2. The binary model, on the other hand, captures the correct scaling behaviour when the viscosity ratio between the two phases is sufficiently high.

Drop dynamics on hydrophobic and superhydrophobic surfaces

We investigate the dynamics of micron-scale drops pushed across a hydrophobic or superhydrophobic surface. The velocity profile across the drop varies from quadratic to linear with increasing height, indicating a crossover from a sliding to a rolling motion. We identify a mesoscopic slip capillary number which depends only on the motion of the contact line and the shape of the drop, and show that the angular velocity of the rolling increases with increasing viscosity. For drops on superhydrophobic surfaces we argue that a tank treading advance from post to post replaces the diffusive relaxation that allows the contact line to move on smooth surfaces. Hence drops move on superhydrophobic surfaces more quickly than on smooth geometries.

Defect motifs for constant mean curvature surfaces.

The energy landscapes of electrostatically charged particles embedded on constant mean curvature surfaces are analyzed for a wide range of system size, curvature, and interaction potentials. The surfaces are taken to be rigid, and the basin-hopping method is used to locate the putative global minimum structures. The defect motifs favored by potential energy agree with experimental observations for colloidal systems: extended defects (scars and pleats) for weakly positive and negative Gaussian curvatures, and isolated defects for strongly negative Gaussian curvatures. Near the phase boundary between these regimes, the two motifs are in strong competition, as evidenced from the appearance of distinct funnels in the potential energy landscape. We also report a novel defect motif consisting of pentagon pairs.